The present invention relates to semiconductor devices and, more particularly, to three-terminal power device, such as discrete transistors and Darlington pairs, formed of identical active elements interconnected in parallel.
Prior art power transistors and Darlington-connected transistors are known to include 3 or 4 semiconductor chips in a single housing, each of which constitutes a functionally complete component, with their terminals interconnected in parallel to three wires which protrude from the housing and constitute the terminals of the device. In FIG. 1 of the accompanying drawings, this type of device is illustrated schematically in plan view.
Three identical semiconductor elements 1, e.g. three transistors having their collector electrodes on the bottom of the chip and their base and emitter electrodes on the top thereof, are soldered to a metal plate 2 having a lug 2' located on one side thereof. Six wires 3 each have one end soldered to the base contact areas 1' or to the emitters of the transistors 1 and each have their other end soldered to two metal strips 4 and 5 that constitute the base and emitter terminals of the device. A housing represented in the drawing by a rectangle 6 holds all the elements of the device except for a portion of the lug 2' of the metal plate 2 and an end portion of each of the strips 4 and 5. The housing may be a solid block of plastic obtained by means of a molding process known in the prior art, or a metal encapsulation properly filled with an inert gas and sealed hermetically. In this instance, the lug 2' and the end portions of the strips 4 and 5 are electrically insulated from the metal encapsulation. In some prior art devices, the metal plate 2 forms part of the bottom wall of the device and is adapted to be mechanically connected to a large metal body not shown in the figure for the necessary outward dissipation of the heat produced by the device during its operation or, in other prior art devices, is mechanically connected to a solid metal plate by means of an electrically insulating, yet thermally conducting, layer, e.g., of beryllium oxide. It will be understood that in the first instance, the connection between the collector of the device and the external circuit is established by the heat sink, so that the lug 2' of the plate 2 is unnecessary, while in the second instance, said lug is indispensable as the external electrical connection.
Prior art devices of this type are capable of operating at power levels of up to 500 Watts. In order to handle higher power levels by the use of a single device, as is being applied in the industry to an ever-increasing extent, it would be necessary to increase the number of semiconductor chips constituting the elementary components of the device. However, this would unduly increase the length of the device if the geometry depicted in FIG. 1 is to be maintained, or it would result in very large and complex structures if one is to interconnect in parallel several structures of the type shown. In either case, assembly with automatic equipment would create problems because of the large number of precision soldering operations needed to connect the chips to the metal strips of emitter and base. Finally, when designing such a device, one should bear in mind the necessity of minimizing the length of the connecting wires and, above all, keeping the length constant so as not to change the uniformity of activation of the single components by the introduction of several resistors connected in series with the bases and with the emitters of the elementary components.